With the rapid development of Internet services and the diversity and variability of emerging services (such as web browsing, multimedia services, resource downloading, and network conference services), conventional core switching networks have already lagged behind the requirements of users. Therefore, in order to keep pace with the explosive growth of Internet traffic, the increasing demand on bandwidth, and the diversity and burst characteristic of the services, researchers devote themselves to the study of wavelength division multiplexing (WDM) and even dense wavelength division multiplexing (DWDM) transmission. At present, the DWDM technology is able to raise the transmission bandwidth to about 10 Tbit/s, thus meeting the requirement on the bandwidth of a transport network in a long term. As a network switching node needs to convert transmitted data from an optical domain to an electrical domain and then back to the optical domain, the integrated circuit (IC) technology has become a bottleneck that restricts the node processing capability in an optical network. Though the network bandwidth is large enough, the network transmission rate is still limited. Thereby, an optical switching technology emerges as required, i.e., the whole conversion is completed in the optical domain.
Currently, the optical network adopts optical circuit switching (OCS), which is simple in protocol, mature in technology, and easy to implement. OCS is similar to circuit switching, and is connection-oriented, so as to establish a transmission optical path for end-to-end (E2E) traffic flow. However, due to the changeability and burst characteristic of the Internet services, the establishment and release of the optical path are quite frequent. Thus, on account of the dramatic contrast between the overhead of the establishment and maintenance of the optical path and the duration of the optical path connection (usually very short), the bandwidth utilization is rather low.
Optical packet switching (OPS) is a switching mechanism of fine granularity. Similar to conventional packet switching networks, the OPS also adopts a store-and-forward technology. As OPS allows the statistics and multiplexing of network channel bandwidth resources, it is particularly suitable for burst Internet data services. Therefore, OPS is quite promising in the long run. However, currently some problems difficult to handle still exist. First of all, the optical switch technology capable of effectively supporting the OPS is not mature, and meanwhile the switching speed of an optical switching matrix is far from meeting the requirement. Secondly, it is also difficult to perform packet synchronization at the entry of a switch, and thus the conflict between the packets cannot be effectively decreased. Finally, a random access mechanism in essence has not been proposed yet for the optical domain, and the current optical domain cache may only be realized through fibre delay lines (FDLs). The use of the FDLs is also limited by the signal quality and physical space, and may not provide sufficiently flexible and adequate cache. In view of the above, the OPS still needs to be studied for many years before being utilized.
Optical burst switching (OBS) is proposed as a compromise solution between OCS and OPS. Packets arriving at edge nodes in an OBS network are assembled into burst packets according to destination address and quality of service (QoS) for later transmission and switching. Therefore, the processing granularity is improved. Each burst packet is corresponding to a burst header packet (BHP). The BHP enters a core switching network before burst packets, and reserves resources hop by hop at all intermediate nodes that the burst packets need to pass by. As long as an offset time between the burst packets and the BHP is properly set, it is ensured that before corresponding burst packets arrive at each intermediate node, the processing of the BHP and the configuration of the corresponding switching channels have already been completed, thereby realizing E2E all-optical transparent transmission of the burst packets. Compared with the OCS, the OBS has a moderate switching granularity, and allows the statistics and multiplexing, thereby raising the resource utilization. Compared with the OPS, the OBS is easier to implement, and has a lower requirement for the components. Therefore, though the OBS is immature in terms of standard and protocol, under continuous research and development, it is likely to become a core technology for optical transmission and switching network in the next generation.
Burst assembly is critical to the design of the OBS network, which greatly affects the performance of the OBS network. FIG. 1 is a schematic view showing the assembly of IP packets into burst packets at edge nodes in an OBS network in the conventional art. Referring to FIG. 1, after arriving at the edge nodes in the OBS network, multiple IP packets are classified according to the destination address and priority thereof, and then sent to corresponding assembly queues. When triggering conditions for assembly are satisfied, the IP packets in the corresponding queues are assembled into a burst packet, then scheduled, and sent to an OBS core network for transmission.
The burst assembly method adopted by the edge nodes in the OBS network determines the triggering conditions for assembly, and meanwhile determines the characteristics of the output burst packets, thereby directly affecting the length of the burst packets, packet lost rate of the burst packets in transmission, E2E delay of the IP data packets, and utilization of the network channels. Therefore, a desired burst assembly method may effectively control the E2E delay of the packets, meet the delay requirements of the services, raise the resource utilization in the OBS network, and decrease the packet lost rate.
Currently, three burst assembly methods are disclosed in the conventional art, which are an assembly method based on a fixed time threshold, an assembly method based on a fixed length threshold, and a threshold adaptive assembly method. These methods are respectively introduced as follows.
A burst assembly method based on a fixed time threshold is disclosed in the conventional art. The basic principle of the method is that, a maximum waiting time T is set for each cache queue, and IP data packets arriving within the time T are cached in corresponding assembly queues. A timer starts timing from a first IP packet entering the cache queue, and when reaching the time T, all the data packets in the corresponding queues are assembled into a burst packet. On account of the blocking of control channels, utilization of the channels, and processing speed of core nodes in an OBS network, the length of the burst packet should have a lower limit, and thus the method further restricts a minimum length of the burst packet. If the assembly time reaches the maximum waiting time T while the burst packet length is inadequate, the packet has to be filled to the minimum length and transmitted later. This method is advantageous in that the E2E delay of the IP data packets can be controlled accurately.
Seen from the above, the burst assembly method based on a fixed time threshold has the following disadvantages.
First, when a certain assembly queue is under a low load, a large amount of data will be filled, and thus the utilization of the burst packet is decreased. When the load is high, a great many data packets will arrive during the assembly, and thus the assembly queue requires plenty of cache. Besides, an extremely long burst packet may also lead to the increase of the packet lost rate. The reason is that, when a conflict occurs, the loss of one burst packet means the loss of plenty of IP packets. In addition, with dynamic changes of the load, the length variations of the burst packets are great, which may affect the network performance.
Secondly, when each edge node adopts the same time threshold, each assembly queue periodically generates burst packets. FIG. 2 is a schematic view showing continuous conflicts caused by assembly synchronization at the edge nodes. In FIG. 2, as each assembly queue at the edge node periodically generates burst packets, assembly synchronization occurs between the burst packets, thereby resulting in continuous resource competitions at an egress of a switching structure of the edge node and the core node. At the edge node, packet loss may be avoided through cache, but an additional cache delay may be generated. However, at the core node, due to the lack of optical cache, a large amount of packet loss may be caused continuously, and the channel resources may not be fully utilized.
A burst assembly method based on a fixed length threshold is disclosed in the conventional art. The basic principle of the method is that, a fixed length threshold is set for each assembly queue, and arriving data packets are still sent to corresponding assembly queues for cache. When a total length of the data packets in the cache queues reaches a length threshold, all the data packets in the corresponding queues are assembled into a burst packet. The method is advantageous in that the burst packet length is relatively fixed, which may improve the network performance, optimize the scheduling, and avoid assembly synchronization to some extent. Compared with the burst assembly method based on a fixed time threshold, the method based on a fixed length threshold is advantageous in decreasing the packet lost rate.
However, the burst assembly method based on a fixed length threshold still has the following disadvantages.
First, when a traffic load is low, this method may cause a great assembly delay, and is thus inapplicable to real-time service transmission. An advanced technical solution is also provided in the conventional art, i.e., a time threshold is added to limit the delay, so that the assembly is completed when either of the two thresholds is reached. That is to say, the assembly based on a fixed time threshold is adopted under a low load, and the assembly based on a fixed length threshold is adopted under a high load. This advanced method integrates two thresholds, but still has limited adaptability to the change of the traffic, and the blocking of burst packets caused by assembly synchronization still exists at a low load.
Secondly, when the load is high, a generation period of the burst packets is short. If the length threshold is set too small, plenty of BHPs may flood into the control channel in a short time, thus resulting in the blocking of the control channel. Moreover, due to dynamic changes of the network load, it is difficult to predetermine an optimal length threshold in practice, and thus the delay problem caused by the method is obvious.
In order to enable the assembly threshold to vary with the dynamic changes of the traffic load, a threshold adaptive burst assembly method is further disclosed in the conventional art. The basic principle of the method is to adopt a prolonged length threshold and a fixed time threshold. The fixed time threshold is set to ensure a maximum tolerable delay under a low load, and the prolonged length threshold is set for adjustment according to the current load condition. Moreover, in order to decrease the continuous dramatic changes of the burst packet length, this solution introduces a hysteresis change characteristic, that is, after each assembly is completed, the burst packet length of this assembly is compared to obtain the latest load condition. If it is ensured that the traffic does change through several comparisons, the length threshold is adjusted.
Seen from the above, the threshold adaptive burst assembly method has the following disadvantages.
First, in this method, if the length threshold triggers an assembly, the load will be increased, and thus the length threshold needs to be enlarged. This inappropriate load balancing method may result in an ever increasing length threshold till the time taken to complete the assembly under the current load reaches the fixed time threshold, or the length threshold reaches a predetermined tolerable maximum value, which may cause an unnecessary increase in the assembly delay. Moreover, the judging whether the load changes or not actually depends on the length of the IP packet that enters the queue last to trigger the assembly. The load determination mode adopted by this method cannot be well adapted to the change of the load.
Secondly, though the assembly length threshold is set to change with the load, this method does not clearly state in what relation the two may match up with each other, i.e., the objective of the adaptability is not explicit.
Thirdly, this method introduces the hysteresis change characteristic, so that the assembly threshold is adjusted after it is ensured that the load changes through several comparisons, thereby decreasing the continuous dramatic changes of the length threshold to some extent. However, in this manner, the tracking ability of the length threshold on the burst is also decreased, i.e., the adaptability of this method is lowered.